Green Grass versus Greenhouse Gas: Using compost to capture carbon
An hour north of the Golden Gate, the rolling grasslands of West Marin are dotted with bay laurels, gnarled oak trees, and cows. In this timeless landscape, locals are ranching an entirely modern commodity—carbon—alongside their livestock.
Whendee Silver, a professor of environmental science, policy and management, is helping ranchers employ their pastures to slow climate change through a project with the Marin Carbon Project, a consortium of researchers, landowners, government agencies and others. Her idea is to pull carbon dioxide out of the atmosphere, where it can warm global climate, and store it in plants and soils. Her secret weapon: compost.
Compost is both a rich source of carbon and a potent fertilizer. As it decays, it makes the nutrients locked in animal manure and vegetable matter available to growing plants. Silver is betting that compost spread over rangelands will turbocharge grass growth and slow atmospheric warming at the same time.
The trick, however, is to control how fast the compost decays. The soil microbes that break down compost into fertilizer also produce carbon dioxide. An ecosystem ecologist, Silver is tuning this process to favor plant production over greenhouse gas emissions.
At the Nicasio Native Grass Ranch, Silver has divided fields into experimental plots. Some have had a thin layer of compost spread on top. With chambers resembling upside-down buckets, Silver can measure how much carbon dioxide and other greenhouse gases each plot emits to the atmosphere.
So far, Silver’s hunch has been right on target. The plots covered with compost absorb far more carbon, are topped with thicker, greener tufts of grass, and produce 50 percent more forage than compost-free control plots. In addition to being a source of fertilizer, the coating of compost helps cool the soil. The lower temperatures put a damper on microbial decomposition.
Rangelands are ideal places to stash carbon, according to Silver. Grasses in seasonally dry areas must grow deep roots in order to find water. For that reason, grasses must store proportionally more of their total biomass—and therefore more of their carbon—underground than plants in moister climates.
If the technique proves successful, ranchers who compost could make some extra money in the future. Polluters might be able to buy ranchers’ carbon scrubbing services in so-called carbon markets.